Spawn Parked Vehicles (Natural)

Hey guys I've been doing some digging regarding anything that controls the types of vehicles that spawn within parking spaces around LS.

I know how to modify the popgroups to spawn certain vehicles in traffic, however I cannot figure out what could control the natural spawn of parked vehicles.

I could create a map with the map editor but well, that defeats the purpose of the natural spawn concept.

Side question: In SP you'll come across random LSPD and sheriff vehicles cruising around the city and Blaine county. How do I add other emergency vehicles to patrol naturally around the game? This is another thing not controlled by the popgroups I believe...